There is an increasing need to allow old and/or vulnerable home dwellers to authenticate legitimate tradespersons or professional careworkers or helpers before allowing them access to their homes, to guard against the possibility of intruders deceiving the homeowner into letting them into their property under false pretences.
As would-be intruders become more sophisticated in their means of deception, methods such as requiring the visitor to display some form of identification is increasingly being seen as insufficient evidence on which to allow the visitor entry. As an additional form of authentication, most legitimate representatives of service companies will provide a telephone number of the organisation of which they are a representative, so that the householder may contact the organisation and verify the identity of the representative. However, this system is somewhat cumbersome to implement in practice and is not completely secure. (How does the househoulder know that the phone number is correct? How can the householder be sure that it is not an imposter who has stolen the legitimate representative's ID, etc.)